The AirDog II incorporates the demand flow. This means that it returns a very small amount of fuel back to the tank. Any unused fuel is recirculated within the base of the unit rather than just being pump back to the tank to foam up yet again. This small amount of fuel is taking the entrained air back to the tank rather then running to your engine. The AirDog II 200 also has an adjustable fuel pressure regulator that you can set yourself for your application. Another feature is the high flow fuel module upgrade kit which eliminates the 1/4 tank issues associated with the installation of bigger suction tubes into the tank. The return flushes back to the fuel basket to provide adequate fuel that your AirDog II needs. All model AirDogs come with 1/2” fuel lines for maximum fuel flow with the least restriction. The All New Fuel Preporator Demand Flow system, with built-in pressure regulator, draws from the tank only the fuel flow necessary to supply the needs of the engine. Returns air & foam to the tank. Max flow of 200gph @ 5psi. FREE Shipping!